The inspection tools in Semiconductors Industry help the manufactures to find and alert about defects or particle contamination in the wafers. Since the tests are nondestructive, the inspection tool should not add any kind of contamination, specifically particle contamination on the wafer.
The environment requirement of the wafer is harder to achieve due the continuous shrinking of the design rule of the wafers. This is specified by ISO 14644-1 cleanroom standards:
Maximum particle per square meterFED≧0.1≧0.2≧0.3≧0.5≧1STD 209EClassmicronmicronmicronmicronmicronequivalentISO1102ISO210024104ISO31000237102358Class 1ISO4100002370102035283Class 10
For modern tools the requirement is ISO class1.
It is important to note the real requirement of wafer manufacturers. What they really want is to have no particles on the wafer. This is measured by PWP (particles per wafer path) indicated by number of particles added to the wafer during the pass of the wafer in the tool. For example 0.25 PWP which means less than 1 particle is added for each 4 wafer passes.
The typical design to control the cleanliness of the tool is using a controlled minienvironment. The minienvironment consist of box containing the critical parts of the tools where a constant laminar gas flow of 0.35 m/s is forced to go vertically downwards. The gas is previously filtered by a high performance ULPA filter. In this way the wafer is exposed to particle free gas. The problem with this concept is that the flow to the wafer is normally obstructed by an optical bench which is a massive construction. In addition the wafer is normally placed on an X-Y stage which can produce particles by itself when performing the scanning U.S. Pat. No. 7,522,258 describes a lithographic apparatus and device manufacturing method utilizing movement of clean air to reduce contamination.